1. Field of the Invention
The invention generally relates to a communications system. More particularly, the invention relates to a communications device and a method of receiving radio frequency signals within a communications system.
2. Description of the Related Art
One example of a communications system is a wireless communications system which may be a cellular mobile communications system. The cellular mobile communications system is implemented in a geographical area and logically divided into individual service cells. A fixed transceiver station such as a base station defines at least one cell and is connected to a base station controller. Mobile stations, such as hand-held or car-based cellular phones, move freely within the geographical area covered by a cell.
The base station handles all telephone traffic to and from those cellular phones which are currently located in the cell. In addition, the cellular phones and the base stations exchange radio signals in accordance with a communications protocol defined for a given communications system. For instance, in a conventional Code Division Multiple Access (CDMA) systems, a traffic channel, a pilot channel, and a paging channel are defined for communications between the base stations and the cellular phones. The pilot channel carries no information, but provides the cellular phone, for example, with a reference for time, phase and signal strength. The cellular phone constantly evaluates the strengths of the pilot channels of the serving and neighboring base stations to determine potential base stations should the cellular phone move from one cell to another.
In some cellular phones, a receiver includes two frequency conversion stages. A first conversion stage down converts a received radio frequency (RF signal) to an intermediate frequency (IF) signal whose intermediate frequency (IF) is lower than the radio frequency. An amplifier amplifies the IF signal and a second conversion stage down converts the amplified IF signal to the baseband.
Conventional direct conversion (DC) module receivers, on the other hand, convert the RF signal directly down to the baseband. The DC module outputs an inphase (I) signal component and a quadrature (Q) signal component which are, in turn, processed by a baseband processor. Subsequent to the downconversion, amplifiers amplify the signal components I and Q in separate I- and Q-channels. The amplifiers should amplify the signal components I and Q with gains that are ideally the same for each one of the signal components I and Q. Unfortunately, gain imbalances between the I- and Q-channels may degrade the signal demodulation in the baseband processor.
An aspect of the invention relates to a method of enhancing the direct conversion of a radio frequency signal. The method comprises converting a radio frequency (RF) signal into an I signal and a Q signal. The method further comprises varying the amplification of the I signal based on the comparison of the amplified I signal, a reference voltage, and a signal strength signal. The method also comprises varying the amplification of the Q signal based on the comparison of the amplified Q signal, the reference voltage and the received signal strength indicator signal.
An aspect of the invention involves a direct conversion receiver for a communications system. The receiver includes a frequency conversion circuit and a control circuit connected to the frequency conversion circuit. The frequency conversion circuit converts a radio frequency (RF) signal into a first signal component and a second signal component which occupy a baseband. The control circuit comprises first ports connected to the frequency conversion circuit to receive the first and second signal components and second ports connectable to a processor circuit to output amplified first and second signal components. Between the first and second ports exist separate channels for the first and second signal components. Each channel comprises an amplifier and a feedback loop to control the amplifier as a function of a reference signal and a control signal derived from the RF signal.
An aspect of the invention involves a wireless communications device that includes a frequency conversion circuit and a control circuit. An antenna receives a radio signal at a first frequency and converts it into an electrical radio frequency (RF) signal. The frequency conversion circuit is associated with the antenna and configured to convert the RF signal into a first signal component and a second signal component. The first and second signal components occupy a baseband. The control circuit has first ports connected to the frequency conversion circuit to receive the first and second signal components and second ports connected to a processor circuit to output amplified first and second signal components, and separate channels for the first and second signal components existing between the first and second ports. Each channel comprises an amplifier and a feedback loop configured to control the amplifier as a function of a reference signal and a control signal derived from the RF signal.
Another aspect of the invention involves an apparatus comprising a frequency conversion circuit and a control circuit. The frequency conversion circuit is configured to convert a first signal having a first frequency into a first signal component and a second signal component. The first and second signal components have a second frequency which is lower than the first frequency. The control circuit has first ports connected to the frequency conversion circuit to receive the first and second signal components and second ports connectable to a processor circuit to output amplified first and second signal components. The control circuit further has separate channels for the first and second signal components existing between the first and second ports. Each channel comprises an amplifier and a feedback loop configured to control the amplifier as a function of a reference signal and a control signal.
A further aspect of the invention involves a method of controlling power levels of first and second signal components, which are processed in separate channels. For each channel, one of the first and second signal components is amplified and a detector signal, which is indicative of a power level of one of the amplified first and second signal components, is generated. Further, an error signal, which is indicative of a difference between one of the amplified first and second signal components and a reference signal, is generated. The error signal and a control signal are summed to generate a gain control signal, and to adjust amplification of one of the first and second signal components.
Another aspect of the invention involves a method of controlling power levels of first and second signal components. The first and second signal components are processed in separate channels. In each channel one of the first and second signal components is amplified and amplification is controlled as a function of an error signal, which is indicative of a difference between the amplified signal component and a reference signal, and a sum of the error signal and a control signal.
For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. Of course, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.
These and other aspects, advantages, and novel features of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings.
FIG. 1 is an illustration of a cellular phone.
FIG. 2 is an illustration of a receive path of a cellular phone.
FIG. 3 is a block diagram of a receiver.
FIG. 4 is an illustration of a frequency conversion module.
FIG. 5 is an illustration of a gain control module.